Generally, in a direct sequence CDMA system, a first spreading code group common to base stations which have the same repetition period as that of an information symbol period (hereinafter, a code which has the same repetition period as the information symbol period will be called a short code) and a second spreading code group which has a longer repetition period than the information symbol period (hereinafter, a code which has a longer repetition period than the information symbol period will be called a long code) are used. A signal is transmitted by spreading doubly with a first spreading code of the first spreading code group and a second spreading code of the second spreading code group (here, the second spreading code varies from one base station to another). The second spreading code is used in order to reduce interference from other base stations since the number of first spreading code of the first spreading code group is limited.
FIG. 1 shows a configuration example of the direct sequence system used in general. In the system, received information is first-spread with a first spreading code. After that, the received information is second-spread with a second spreading code. Information which is applied to an input terminal 1 is first-spread in a multiplier 2 by multiplying the information by an output from a first spreading code generator 3. Next, a spreading output from the multiplier 2 is multiplied by an output from a second spreading code generator 5 such that second-spreading is performed and a spreading modulation signal output is obtained at an output terminal 6.
FIG. 2 shows another configuration example. According to the configuration, the received information is spread by performing an exclusive OR operation on the first spreading code and the second spreading code. A result of an exclusive OR operation of an output from the first spreading code generator 3 and an output from the second spreading code generator 5 is calculated by an exclusive OR circuit 9. Information received at the input terminal 1 is calculated using the result of the exclusive OR operation in a multiplier 8 such that the spreading modulation signal output is obtained at the output terminal 6.
Here, an orthogonal code (for example, a GOLD code) is used as the first spreading code generally. The number of the orthogonal codes which are generated is limited to the number of a spreading ratio. Therefore, in a general direct sequence CDMA communication system, a signal is spread and transmitted with the second spreading code having a longer repetition period than the information symbol period as well as the first spreading code having a repetition period of the information symbol period. The second spreading code having the longer repetition period can have a large number of spreading codes by highly increasing the repetition period.
FIG. 3 shows a configuration example of a receiver where a signal is transmitted after being spread doubly with a first spreading code and a second spreading code.
A signal which is doubly spread with the first spreading code and the second spreading code is received by an antenna part 10 and received by an RF receive part 11. The received signal is multiplied by an output from a second spreading code generator 13 in a multiplier 12 such that the received signal is first-despread. Next, an output from the multiplier 12 which is despread is multiplied by an output from a first spreading code generator 15 in a multiplier 14 such that it is second-despread, and then an output from the multiplier 14 is applied to a demodulator 16. After that, demodulated data is obtained from the demodulator 16.
Instead of despreading by using the multiplier 12 and the multiplier 14 with the second spreading code and the first spreading code, respectively, despreading can be carried out by using a multiplier with a result of an EXOR operation on the first spreading code and the second spreading code as shown in FIG. 2.
FIG. 4 shows a configuration example of a transmitter where a signal is spread doubly with a first spreading code and a second spreading code, and is sent.
Data to be transmitted is applied to a modulator 21. An output from the modulator 21 is multiplied by an output from the first spreading code generator 23 such that it is first-spread. Next, an output from a multiplier 22 which is spread is multiplied by an output from a second spreading code generator 25 in a multiplier 24 such that it is second-spread. Then, it is applied to an RF transmitter 26. An output from the RF transmitter 26 is output from an antenna part 20.
Instead of despreading by the multiplier 22 and the multiplier 24 with the first spreading code and the second spreading code, despreading can be carried out by using a multiplier with a result of an EXOR operation on the first spreading code and the second spreading code as shown in FIG. 2.
By the way, in a wireless mobile communication system, a mobile station communicates with a telephone terminal in a public network or the like via a wireless base station. A wireless circuit can be easily listened in on or used fraudulently since the circuit is open to the air. Hence, there have been various technologies conventionally which enable a mobile station to connect to only a specific base station in order to avoid others listening in or a fraudulent use.
For example, Japanese laid-open patent application No. 63-189026 discloses an invention of a cordless telephone system such as a normal domestic cordless telephone. In the cordless telephone, a cordless handset and a cordless base transmit/receive a unique system identifying number (which is configured by a fixed identifying number which is assigned to the cordless base and the cordless handset and a plurality of identifying numbers which determine an order) such that a cordless base which can communicate with a cordless handset is identified. Communication is allowed only when the system identifying number of the cordless base matches with the system identifying number of the cordless handset.
As another example, Japanese laid-open patent application No. 7-203540 discloses an invention regarding a business cordless telephone system which provides a roaming service for a terminal which moves in a wireless service area of a PBX. In the conventional example, a cordless handset stores a plurality of unique system identifying numbers (base station IDs) which are assigned to each system which numbers can be communicated, or the cordless handset stores a network identifying number (network ID) which indicates a plurality of system identifying numbers which can be communicated by the cordless handset. When communicating, the cordless handset identifies a cordless base which can communicate with the cordless handset by transmitting/receiving the identifying numbers. That is, when the system identifying numbers or the network identifying numbers of the cordless handset and the cordless base are the same, the cordless handset and the cordless base can communicate with each other.
FIG. 5 shows an example of a generalized configuration for a roaming service between PBXs (private branch exchanges). When a mobile terminal apparatus 43 moves from a zone of the PBX A 41 to a zone of the PBX B 42, the system identifying number or the network identifying number is checked so as to determine whether the roaming service is provided.
Generally, a control for determining whether a base station and a mobile station are allowed to connect should be carried out as quickly as possible, since the control is a preparation.
By the way, if the inventions disclosed in the above-mentioned Japanese laid-open patent application No. 63-189026 and Japanese laid-open patent application No. 7-203540 are applied as is to the direct sequence CDMA mobile communication system, the mobile station needs to receive and recognize the system identifying number or the network identifying number. For that purpose, the mobile station needs to know a spreading code (a long code or a short code) which the base station uses. After despreading a received signal with the spreading code, the mobile station can know the system identifying number or the network identifying number from an information symbol. In this case, if the spreading code is not known beforehand, it is necessary to identify the spreading code. Therefore, it takes much time for the mobile station to determine whether the mobile station can connect to the base station from the received signal. Thus, the above-mentioned method is not practical.